Idiotypic determinants are unique antigenic determinants found on some or all antibodies of a given specificity but lacking from antibodies of a different specificity. In some instances, a substantial fraction of antibodies of a given specificity possess similar idiotypic determinants, which are referred to as cross reactive idiotypes (IdX's). This research program is mainly concerned with the characterization of IdX's expressed by anti-polysaccharide antibodies, most notably anti-bacterial levan (BL) and anti-inulin and the use of these systems to study the regulation of antibody expression. Our results indicated that an anti-IdX in the anti-BL system (E109) can directly suppress expression of B lymphocytes bearing that IdX and can indirectly suppress E109- anti-BL responses. The cellular and genetic factors regulating both direct and indirect suppression have been studied.